High-frequency induction heating apparatus



March 4, 1952 H. F. STORM HIGH-FREQUENCY- INDUCTION HEATING APPARATUS Filed Sept. 25, 1946 2 SHEET SSHEET l Time in Seconds Ati'orneys M33181! 4, H, F. STORM HIGH-FREQUENCY iNDUCTIQN HEATING APPARATUS Filed Sept. 25, 1946 2 SHEETS-SHEET 2 Inventor Herbert F 81mm Patented Mar. 4, 1952 .HIGH-FREQUENCY INDUCTION HEATING q APPARATUS Herbert F. Storm, Scotia, N. .Y., assignor to Sunbeam Corporation, Chicago, 11]., a corporation of Illinois Application September 25,1946, Serial No. 699,197

17 Claims. 1

The present invention relates to a controlarrangement for a high frequency heating device and more particularly to a control arrangement which enables more efficient use of the high frequency generator. Specifically, the present invention is an improvement on the arrangement disclosed and claimed in my copending application, Serial No. 699,196, now Patent Number 2,521,880 filed concurrently with the present application-and assigned to the same assignee as the present application.

Induction heating devices are in extensive use today in-many manufacturing operations. For a given work piece and for a given inductor or high frequency heating coil for heating such work piece, the heat generated in the work piece or charge is a function of the high frequencycurrent flowing in the inductor coil, the frequency of this high frequency current, the permeability of the charge or work piece, and the electrical resistivity of such charge or work piece. The generated heat per unit of time-can be represented by the following equation:

P=Kiwm where P is the heating rate inB. t. u.s per second; K is a constant; 2' is the current in amperes flowing in the inductor coil; 1 is the frequency of thishigh frequency current; is the permeability of the work piece or charge'to be heated; and p, is the resistivity of the work piece in ohms per cubic centimeter. It is quite obvious from this equation, thereforathat the heat generated by induction in a certain work piece depends upon a number of parameters. Since most materials have a positive temperature coefficient with respect to the electrical resistivity thereof and since the above equation shows that the'heat generated by induction is a function of electrical resistivity, it is clear that the heat generated will increase with temperature for material having a positivetemperature coefiicient. In the case of ferromagnetic charges, the permeability is also subject to considerable variation since it is well known that'when the temperature of such material reaches the recalescent point, the magnetic property of the material disappears so that the permeability drops to unity with an attendant drop in heat generation. For many ferromagnetic materials, the permeability with reference to that of air may be hundreds of times greater.

From the'above discussion, it will be apparent that by virtue of thechange in electrical resistivity as well as permeability with temperature-if the highfrequency generator is matched to its load impedance was to deliver maximumpower when supplying .a cold load that it willbe overloaded when the temperature. ofthe work piece increases and vice versa. Heretofore such' high frequency generators were fully utilized only. during part of .the heating cycle. In order'ntoobtainfullpower output or more nearly full. power output during the entire heating cycle, it is necessary to rematch the high frequency generator with its load impedance as the temperatureof the work piece changes.

Accordingly, itis an object of the present invention to provide a new and improvedcontrol arrangement for an inductionheatingdevice-in which the high frequency generator isutilized more fully throughout the entire heating cycle than in priorart arrangements used heretofore.

It is another object of the present invention to provide a new and improved controlarrangement for a high frequency heating device in which the-impedance of theload is automatically varied in response to changes of electrical resistivity and permeability of the workpiece-with changes in temperature.

It is a further object of the present invention to providea new type of-variable inductor-and means for automatically varying the inductance thereof in order to maintain constant the output of a high frequency generator associated description proceeds and the features. of novelty which characterize this invention will be pointed out with particularity in the claims annexed to and forming a part of this:specification.

Fora'better understanding of the-presentinvention, reference may be had to the accompanying drawings in which:

Fig. 1 .is a'curve diagram to aidinunderstanding the present invention;

Fig. 2 is a schematic circuit diagramof a control arrangement embodying the present invention;

Fig. 3 is a view similar to Fig. 2 of .a portion of the control-arrangement of .Fig.'2:illustrating a modification of the present invention; and

Fig.4 is a partial view of the control arrangepass, Pmax.

ment shown in Fig. 3 illustrating still another modification of the present invention.

The present invention is primarily concerned with utilizing the inherent resilience of the water-cooled spirally coiled conducting tube forming the inductance of the tank circuit of a conventional high frequency oscillation generator to vary the curent flowing in the associated inductor or heating coil by changing the axial length of this water-cooled coiled tube. In accordance with specific illustrated embodiments of the present invention, the changes in axial length of this water-cooled coil are accomplished in various manners and furthermore, are automatically accomplished in response to the load on the high frequency generator during the heating cycle so as to utilize this generator to its maximum extent throughout the heating cycle. In one embodiment, the axial length of this coil is varied by means of a cam and in another embodiment, the axial length of this coil is varied in response to a function of the grid current of the electron discharge valve of the oscillation generator. In still another embodiment of the invention, the axial length of the water-cooled inductance of the tank circuit is controlled as a function of the plate current of the oscillation generator.

Referring now to Fig. l of the drawings there is shown a curve A obtained by plotting the rate of heat generation in a particular work piece of ferromagnetic material as a function of time. It will be noted, upon an examination of this curve A, that the rate of induced heat generation varies greatly with time during a heating cycle. As illustrated in Fig. 1, the heating cycle is started with the work piece of ferromagnetic material at room temperature, thereby having considerable permeability. As the heating process progresses, the electrical resistivity of the work piece or charge increases since the material has a positive temperature coefficient, whereby from Equation 1 above the heat generation rate also increases, thus accounting ior the initial rising portion of the curve A. As the temperature of a ferromagnetic work piece continues to increase, it reaches its recalescent point where it changes from a magnetic to a nonmagnetic state with a very great drop in the permeability thereof. This point is represented by the temperature ii in Fig. l of the drawings. Thereafter as the temperature of the charge or work piece continues to increase, the rate of heat generation P increases again. which is represented by the second rising portion of the curve A. In other words, the curve A of Fig. l of the drawings represents the conventional heat generation curve for a charge of ferromagnetic material in which the electrical resistivity and permeability thereof vary with temperature. If an overload of the high frequency generator supplying the induction heating device is to be avoided, adjustments must be made such that the peak load occurring at time t1 does not sur- It is furthermore apparent from an examination of the curve A of Fig. 1 of the drawings that at any time other than the time t1, the high frequency generator supplying the power for .the induction heating cycle is only partially loaded, resulting in only partial utilization of the equipment. In the ensuing discussion there is described a control arrangement which will automatically control the high frequency heating device so that the curve of the rate of heat generation will, unlike the curve A of Fig. 1 of the draw-1 4 ings, tend to approach the line Pmax, thereby to insure full utilization of the high frequency generator throughout substantially the entire heating cycle.

Referring now to Fig. 2 of the drawings, there is illustrated a control circuit for a high frequency induction heating device comprising an inductor or heating coil [0 which usually comprises a one turn water-cooled coil of the type disclosed in my copending application, Serial No. 652,756, filed March 7, 1946, now Patent Number 2,528,798, and assigned to the same assignee as the present application. In order to energize the inductor or high frequency heating coil II] with high frequency current so that the Work piece or charge, not shown, associated therewith is heated in an efficient and high speed manner, there is provided an oscillation generator generally indicated at ll. It should be understood that any suitable means for providing high frequency energy to inductor or heating coil I!) may be employed and accordingly, oscillation generator I I may comprise any standard form of generator for producing high frequency oscillations. In Fig. 2 of the drawings this generator has been illustrated as comprising the well known Colpitts oscillator including an electron discharge valve l2 having an anode or plate l3, a cathode I4 and a control electrode or grid l5. The oscillating action of the'electron discharge valve I2 is controlled by a tuned or resonant circuit generally referred to as a tank circuit comprising a serially arranged inductance l6 and inductor or heating coil iii connected in parallel with a pair of serially arranged capacitors l7 and I8. This tank circuit comprising the elements li], l6, l1 and I8 is connected with electron discharge ,valve I2 in the following manner. The cathode I l of the electron discarge valve [2 'is connected to a point on the tank circuit between the capacitors I! and i8 which act as a voltage divider. The lower terminal of the tank circuit is connected to the control electrode or grid l5 through a grid blocking capacitor l9. This connection from the tank circuit to the control electrode l5 provides the feed-back circuit which is a capacitive feedback circuit. The upper terminal of the tank circuit is connected to the plate or anode l3 through a plate blocking condenser 29 in order to insulate the anode or plate I3 from the control electrode or grid H5 in so far as the direct current potential applied to the anode-cathode circult of the electron discharge Valve I2 is concerned while still maintaining the plate l3 and the upper terminal of the tank circuit at the same high frequency potential. In order to provide the desired bias on the control electrode [5 of the electron discharge valve [2, there is provided a grid leak circuit connected between the cathode i l and the grid I5 which comprises an inductance 2i serially arranged with a resistor 22. Preferably the cathode i4 is grounded as is indicated at 23.

It will be understood that an oscillation gen erator of the type described thus far effectively converts direct current energy to high frequency alternating current energy. Consequently, it is necessary to supply the plate-to-cathode circuit of electron discharge valve l2, which will generally be referred to hereinafter as the input circuit of the oscillation generator I l,withasource of the direct current potential. Any suitable source of direct current potential may be employed. Since ordinary 60 cycle alternating current power 5 isco'znmonly available, there is illustrated in Fig. 2 of the drawings a source of alternating potential 25. Thesource 25is connected through a suitable control device illustrated as an electric circuit breaker or contactor 26 to a suitable rectifier unit 21 in order to convert the alternating current to the direct current required at the input terminals of the oscillation generator II. Rectifier unit 21 is illustrated as a oi-phase rectifier comprising a rectifier transformer 28 including a primary winding 29 and a secondary winding 33 having a mid-tap 3 I. The primary winding 29 is conne'cted to the source 25 through the circuit breaker or contactor 26. The secondary winding 3i), on the other hand, has its outside terminals connected to the anodes 32 of a pair of rectifier tubes 33 and 34, respectively, each provided with a cathode 35. The cathodes 35 are interconnected as is clearly illustrated and in turn are connected through a suitable chol coil 35 to one input terminal of the oscillation generator I I. The mid-point 3I of the secondary winding 3% of the rectifier transformer 23 is connected to the other input terminal of the oscillation generator I I.

A suitable by-p-ass condenser 31 may be provided.

to by-pass the high frequency oscillations and with choke coil 36 to impede the passage of high frequency power into the rectifier circuit 2?.

For the purpose of enabling an operator readily to connect and disconnect the oscillation generator I I with respect to the source of power 25, circuit breaker or contactor 2% is schematically illustrated as of the latched closed type which is normally biased to the open position as illustrated either by gravity or by suitable spring means not shown. A circuit breaker closing winding 4-3 is provided which may be energized by closing a suitable closing control switch 6i connected to closing winding 43 through contacts 52, which are closed when the circuit breaker or contactor 25 is in the open position. This closing control circuit is, of course, connected to a suitable source of control potential. When the circuit breaker orcontactor 25 is moved to the closed position, a suitable latch 43 is biased into latching engagement with a member 44 associatedl with the circuit breaker 25 to maintain the circuit breaker or contactor in the close-d position. A trip coil 45, is provided which may be energized from a suitable source of potential upon closure of the tripping control switch 35 whereupon latch 43 is moved to release circuit breaker 26 and permit it to move to the open position under the influence of gravity or suitable circuit breaker opening springs not shown. The contacts 42 limit the opening movement of the circuit breaker 23. It should be understood that the circuit breaker or contactor 26 schematically illustrated in Fig. 2 of the drawings forms no part of the present invention and is illustrated and described only for the purpose of showing a complete control arrangement.

In order to improve the shape of the curve A of Fig. l of the drawings so that it approaches a straight line coinciding with the dotted line marked Pmax, it is necessary to vary some of the parameters set forth in the Equation 1 above to compensate for the changes in permeability ,u and resistivity p occasioned by the change in temperature of the workpiece or charge associated with the heating coil or inductor Iii. An examination of the equation will indicate that perhaps the simplest way to compensate for changes in resistivity and permeability of the charge or work piece is to change the current flowing in inductor or heating coil I 0. This can be accomplished by varying the impedance of the winding or coil I6 of the tank cricuit of the oscillation generator II. If the inductance of the coi I6 is varied, the frequency of the oscillation generator will change by virtue of the fact that the frequency of the oscillation generator is expressed by the following equation:

2 f hm where L and C represent the inductance in henries and capacitance in farads of the tank circuit of the oscillation generator I I. An examination of this Equation 2 shows that a decrease in the inductance L will cause an increase in the frequency f and an increase in the inductance L will cause a decrease in the frequency f. However, an increase in the inductance of coil Iii will cause a decrease in the current flowing in the inductor or heating coil H! which i in series therewith and a decrease in inductance of the coil It will cause an increase in the current flowing in the inductor or heating coil Iii. An examination of Equation 1 above indicates that the rate of heat generation P varies as the square of the current and only as the square root of the frequency. Consequently, although a decrease in the inductance of winding it will cause an increase in frequency, it also causes an increase in the current i. In view of the further fact that in Equation 1 the rate of heat generation P varies as the square of the current i and only as the square root of the frequency f, the net result is an increase in the power generation P for a decrease in the inductance of coil It.

There are several way in which the inductance of coil it could be varied as for example by the use or" a tap changer or by a variometer. A tap chan er is undesirable, however, by virtue of the fact that arcing will cause undue wear on the contacts. A varioineter on the other hand does not lend itself to watercooling, which is almost required with respect to the winding I6,

2 without introducing considerable mechanical difficulties. In accordance with the present invention, variations in the inductance of windin I6 are obtained by employing the inherent elasticity of the water-cooled copper tube or conductor 1; from which the inductance It is usually made.

The inherent elasticity of such a tube or winding permits its elongation and contraction in a manner similar to that of a coiled spring. If the copper pipe is properly insulated it can be compressed without established contact or shorts between adjacent turns. The inductance of a single layer winding or solenoid can be expressed by the following equation:

where L is the inductance in henries; Z is the axial length of the coil in centimeters; n is the number of turns in the coil or the winding; K is a constant; and a is the radius of the turns or coils.

It i apparent from Equation 3 that an increase in the axial length Z of the coil i6 will cause a decrease in the inductance L, and conversely a decrease in the axial length 1 of the inductance coil IE will cause an increase in the inductance L.

In the embodiment illustrated in Fig. 2 of the drawings, the axial length of the winding I6 is controlled in response to the rotation of a cam 53 which makes one revolution per heating cycle. It will be understood that the lower end of the windin I6 is suitably fastened to a rigid support schematically illustrated at 49. The upper end of the winding I6 is connected to one end of a rod having a cam follower 52 at the other end thereof. Rod 5| is guided for reciprocal movement by means of suitable guides 53 and 54. A plurality of springs 55 11orma1ly bias the rod 5! and consequently the winding iii to its position of minimum axial length or in other words, its position of maximum inductance. The springs 55 furthermore hold cam follower 52 against the engaging surface of rotatable cam 50.

In order to rotate cam 50 once during each heating cycle, there is provided a suitable electric motor generally indicated at 55 which is illustrated schematically as a shunt wound direct current motor. t will be understood, however, that any suitable means for causing cam 59 to make one revolution per heating cycle may be employed drivingly connected to cam 50. In the schematic illustration in Fig. 2 of the drawings, the cam 50 is in the position which it would assume at the beginning of the heating cycle. With the cam in this position, the current in the inductor or heating coil [5 will be a maximum and the greatest permissible rate of heating will occur. As time goes on and the charge is apt to absorb more heat as will be understood from an examination of Fig. 1 of the drawings, the rotation of cam 50 in the direction of the arrow shown in Fig. 2 of the drawings will cause the roller 52 to move downwardly, thus resulting in an increase in the inductance of winding i6 and consequently, a reduction in the current i flowing in inductor HI. The cam 55 is designed particularly for a charge or Work piece of ferromagnetic material and when the recalescent point is reached in the time 151 with reference to Fig. 1 of the drawings, the cam 55 has rotated to the point 55' or in other words, so that the roller 52 reaches the rather abrupt break in the cam surface whereby the roller 52 is moved to its maximum upward position with the consequent lengthening of winding l6 and greatly reduced inductance thereof. Instead of the rate of heat generation dropping very rapidly at this point in accordance with the curve of Fig. l of the drawings, however, the lengthening of winding [6 insures a considerable increase in heat generation. Consequently, the variation in winding 16 at this time counteracts the great change in permeability of the charge or work piece associated with inductor coil 16. As the heating cycle continues, the inductance of winding I6 is again increased by decreasing its axial length until it reaches a new low as indicated at 55" in Fig. 2 of the drawings thereby counteracting the increased load resistance as the heating cycle continues.

It will be understood that suitable control means may be provided for motor 55 to insure its energization to start the rotation of cam 50 at the beginning of each heating cycle. This could readily be accomplished by controlling motor 56 from suitable auxiliary contacts associated with the circuit breaker or contactor 25. It will also be understood that the movable upper end of the variable inductance 16 will be connected to the tank circuit of the oscillation generator H by means of a flexible lead or conductor such as is indicated at 51 in Fig. 2 of the drawings.

In View of the detailed description included above, the operation of the control arrangement described above will be apparent to those skilled in the art. The cam 56 is designed in accordance with the particular work piece or charge heated by the inductor or heating coil 10. Since induc tion heating devices are generally used for repetitive heating operations a control arrangement by means of a cam such as 55 is very satisfactory. Furthermore, with this arrangement, the winding i6 is automatically controlled so as to maintain a substantially constant power output for the oscillation generator H as contrasted with its output shown by the curve of Fig. 1 of the drawings if the present invention were not employed.

Instead of controlling the axial length of the water-cooled inductance l6 by means of a cam as illustrated in Fig. 2 of the drawings, suitable automatic control means responsive to some characteristic or function of the oscillation generator II may be employed. Such an arrangement is shown in Fig. 3 of the drawings where only a portion of the control circuit of Fig. 2 is illustrated. However, since the major portion thereof is identical with that of Fig. 2 of the drawings, some of it has been omitted and as far as the portion thereof shown in Fig. 3 of the drawings which corresponds to Fig. 2 is concerned, the same reference numerals are applied to corresponding parts. The rotatable cam 56 has been replaced by a pinion 59 connected to motor 56 which is illustrated as a direct current motor having a constant field excitation. The pinion 59 engages with a rack portion 60 formed at the end of the rod 5! not connected to the winding [6 which otherwise is identical with the rod 5! of Fig. 2 of the drawings.

In accordance with the embodiment illustrated in Fig. 3 of the drawings, the direct current motor 56 is controlled by means responsive to a function of the grid current of the electron discharge valve l2 of the oscillation generator ll. Energization of the motor 56 causes the pinion 59 to rotate in such a direction as to move the rod 5! upwardly against the bias of the springs 55. With this arrangement, the motor 56 and the springs 55 oppose each other and whichever applies the largest force to rod 5| predominates and causes the inductance 16 to either lengthen or shorten axially.

To operate motor 55 in this manner, the armature circuit thereof is energized from a source of direct current potential represented by battery 5|, one terminal of which is connected to one terminal of the armature of motor 56 while the other terminal is connected to the grounded cathode of electron discharge valve l2. The other terminal of the armature of motor 56 is connected through an electron discharge valve 63 to a suitable variable tap 65 on the grid leak resistor 22. Thus, the grid circuit of electron discharge Valve 12 is connected in the armature circuit of the motor 56. As illustrated, the electron discharge valve 63 comprises an anode 65, a cathode 66 and a control electrode or grid 61. The cathode 66 is connected to the variable tap 6 3 while the plate or anode 65 is connected to one terminal of the armature of motor 56. The control electrode or grid 61 of electron discharge valve 63 is supplied with a suitable bias voltage from a battery 58 having connected across the terminals thereof a potentiometer 69. One terminal of the battery and one terminal of the potentiometer are grounded as indicated at 1B. A Variable tap H on the potentiometer 69 is connected to control electrode or grid 61 through a suitable current limiting resistor 12. A suitable by-pass capacitor 13 may be provided between ground 23 and the variable tap 64 associated with the grid leak resistor 22. It will be understood that if the load on oscillation generator II exceeds its permissible value, the grid current of the oscillation generator I I will decrease below a predetermined value with the result that the potential of cathode $3 of electron discharge valve 63 becomes less negative with respect to ground. Since the electron discharge valve is provided with a fixed negative'bias, the potential of the grid 6'! thereby becomes relatively more negative. Consequently, if the grid 61 becomes relatively more negative, the plate current flowing through the electron discharge valve 63 is reduced and consequently, the current supplied to the armature of the motor 56 is reduced. As a result, the motor 58 exerts a decreased torque on the rod 5! and the restraining springs 55 apply the predominant force to the rod 5! thereby reducing the axial length of the winding I6 with the consequent reduction of current by virtue of the increased inductance afforded by the winding I6. If, on the other hand, the load on the oscillation generator I I should decrease, the various elements would operate in just the opposite manner, resulting in an increase in the axial length of the variable inductance It to increase the load on the oscillation generator II and bring about the desired regulation.

In accordance with still another embodiment of the present invention illu trated in Fig. 4 of the drawings, the variable inductance it may be controlled as a function of the plate current of the rectifier unit 2?. The corresponding parts of Fig. 4 are designated by the same reference numerals as in Fig. 3 of the drawings. The electron discharge valve 63 has its cathode 66 connected directly to the grounded terminal of the battery 6i, thereby completing the plate circuit of this valve through the armature of the motor 58 and the battery GI without being connected to the grid circuit of the valve I2 as in Fig. 3 of the drawings. The control electrode or grid 61 of the electron discharge valve 63, however, is connected through a current limiting resistor M to a variable tap I5 associated with a potentiometer i=3 whose terminals are connected across a suitable source of direct current potential 'Il. One terminal of potentiometer I6 is also connected through a variable tap I8 with a resistor I5 connected in the plate circuit of the electron discharge valve I2 or in other words, connected in series with the input circuit to o cillation generator I I. A capacitor 86 may be connected across the resistor 59 if desired to by-pass the high frequency component of potential. Actually, as illustrated, the resistor '59 is connected between the cathode I i of electron discharge valve I2 and the mid-tap 3%! of the secondary Winding 30 of the rectifier transformer 28. The signal picked up by the resistor I9 is obviously a function of the plate current of the electron discharge valve it. By balancing this signal against a voltage obtained from the direct current source or battery I! on the potentiometer I6, the same operation of the motor 56 will be obtained as discussed in Fig. 3 of the drawings. If, for instance, the oscillation generator I I is insufiiciently loaded, the predominating voltage applied to the grid or control electrode 61 of the electron discharge valve 63 will be that supplied by the bias battery I! and this is adjusted so as to be more positive, the more predominant the bias voltage of the battery I? is. As a result, the electron discharge valve t3 will be rendered more conductive thereby increasing the torque on the motor 56 with the resultant decrease in inductance of the'winding IE. More current will flow through the inductor II] as a consequent of this to increase the rate of heat generation and consequently the load on oscillation generator I I which is the desired regulation. In the event that the oscillation generator I I becomes overloaded, the signal picked up at resistor 2'9 will predominate over the voltage of battery 'II, thus tending torender the control electrode 6i more negative and resulting in rendering the electron discharge valve 63 less conductive. In this case, the springs 55 will produce the predominant force acting on rod 5I with the resultant decrease of current flowing through inductor winding I'il.

While there have been shown and described particular embodiments of the present invention as applied to a high frequency heating apparatus, it isto be understood'that the arrangements disclosed-are merely illustrative of the invention. It will, of course, be apparent to those skilled in the art that changes and modifications may be made without departing from the-present invention-and it is aimed in the appended claims to cover all'such changes and modifications-as'fall within the true spirit and scope of the present invention.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. In a high frequency heating device, an oscillation generator including atank circuit comprising a water-cooled inductance formed of a tube of conducting material having suflicient inherent elasticity so that the axial length thereof may be controlled, and means for varying-the axial length of said inductance comprising a rotatable cam having a configuration representative of the heat ab orbing capabilities during a heating cycle of a charge to be heated by said heating device whereby the output of said generator may be controlled in a predetermined manner duringa heating cycle.

2. Ina high frequency heating deviceof the type comprising an oscillation generator supplying a'variable impedance load including an electron discharge valve and a tank circu t. said tank circuit comprising an inductance in the form of .a winding whose axial length is variable, and means responsive to the plate circuit current of said electron di charge valve for automatically varying the axial length of said winding continuou ly to match the impedance of the load on said'oscillation generation with the impedance of said generator.

3. In a high frequency heating deviceof the type comprising an oscillation generator supplyinga variable impedance load including anelectron discharge valve and a tank circuit, the-combination of an inductance in said tankcircuit in the formof a winding whose axial length is variable, and means responsive to acurrent characteristic of said electron discharge valve for automatically varying the axial length of said winding continuou ly to match the impedance of the load on said osc llat on generation with the impedance of said generator. 1

4. In a high frequency heating device of the typecomprising an oscillation generator supplying a variable impedance loadincluding an electron discharge valve and a tank circuit, the combination of .an inductance in said tankcircuit inthe form of .a winding whose .axial length is variabla'means responsive to the grid current of said electron .discharge valve forautomatically varying theaxial length of said .windingcontinuously to match the impedance of the load on said oscillation generation with the impedance of said generator.

5. In a high frequency heating device comprising an oscillation generator, an electron discharge valve including an anode, a cathode and a control electrode, a tank circuit associated with said electron discharge valve comprising an inductor and a variable inductance in the form of a water-cooled helical winding whose axial length is variable to vary the inductance thereof, means for holding one end of said inductance in the fixed position, a reciprocal rod connected to the other end of said inductance, spring means normally biasing said inductance to its minimum axial length, and means operative in response to the heat absorbing capabilities of a charge heated by said heating device for moving said reciprocating rod against the bias of said spring whereby the current flowing in said inductor is automatically controlled.

6. In a high frequency heating device comprising an oscillation generator, an electron discharge valve including an anode, a cathode and a control electrode, a tank circuit associated with said electron discharge valve comprising an inductorand a variable inductance in the form of a water cooled helical winding whose axial length is variable to vary the inductance thereof, means for holding one of said inductance in the fixed position, a reciprocal rod connected to the other end of said inductance, spring means normally biasing said inductance to its minimum axial length, and a motor driven cam for moving said reciprocal rod in dependance upon the heat absorbing capabilities of a charge heated by said heating device during the course of a heating cycle.

'I. In a high frequency heating device comprising an oscillation generator, an electron discharge valve including an anode, a cathode and a control electrode, a tank circuit associated with said electron discharge valve comprising an inductor and a variable inductance in the form of a water-cooled helical winding whose axial length is variable to vary the inductance thereof, means for holding one end of said inductance in the fixed position, a reciprocal rod connected to the other end of said inductance, spring means normally biasing said inductance to its minimum axial length, a rack portion on said reciprocal rod, a pinion engaging said rack, a motor for rotating said pinion to move said rack and reciprocal rod against the bias of said spring means to increase the axial length of said inductance, and means responsive to a characteristic of said oscillation generator for controlling the energization of said motor.

8. In a high frequency induction heating device of the type including a heating coil, an

oscillation generator connected to supply said.

heating coil with high frequency oscillations and a tank circuit for said oscillation generator in which said heating coil forms a part of said tank circuit, the combination of an inductance in said tank circuit comprising a helically wound conducting tube having suflicient inherent elasticity so as to spring back to its initial length upon release of a force applied to reduce its axial length, spring means for maintaining the axial length of said helically wound tube at a minimum, and means responsive to a predetermined current characteristic of said o cillation generator to produce a fork in oppo ition to said spring means whereby the axial length of said helically wound tube is varied to control the current flowing in said heating coil and said oscillation generator is utilized to its maximum efficiency without exceeding a predetermined maximum load thereon.

9. In a control arrangement for a high frequency heating device for a ferromagnetic work piece comprising a high frequency generator including a tank circuit having an inductance in the form of a resilient helical coil, and rotatable cam means for varying the axial length of said coil to vary the output of said high frequency generator, said cam means having such a configuration to tend to maintain constant the output of said generator including an abrupt portion thereon corresponding to the recalescent point of said ferromagnetic work piece.

10. In a high frequency induction heating device of the type comprising a heating coil, an OS- cillation generator connected to supply said heating coil with high frequency oscillations and a tank circuit including said heating coil, the combination of an inductance coil connected in said tank circuit, said inductance comprising a helically wound conductor having sufiicient inherent elasticity so as to spring back to its initial length upon release of a force applied to reduce the axial length of said inductance coil, spring means for maintaining the axial length of said inductance coil at a minimum, and means operative during a heating cycle of said heating device for producing a predetermined variable force in opposition to said spring means so that the axial length of said inductance coil is varied to control the current flowing in said heating coil.

11. For use in a high frequency induction heating device of the type comprising a heating coil, an oscillation generator connected to supply said heating coil with high frequency oscillations and a tank circuit for said oscillation generator in which said heating coil forms a part of said tank circuit, the combination of an inductance in said tank circuit comprising a winding formed of a helically wound conducting tube having sufficient inherent elasticity so that said winding springs back to its unrestrained axial length upon release of a force applied to reduce its axial length, spring means for maintaining the' axial length of said winding at a minimum, and means responsive to a current characteristic of said oscillation generator during a heating cycle to produce a variable force in opposition to said spring means so that the axial length of said winding is varied to control the current flowing in said heating coil whereby said oscillation generator is utilized to its maximum efficiency without exceeding a predetermined maximum load thereon.

12. For use in a high frequency induction heating device of the type comprising a heating coil, an oscillation generator connected to supply said heating coil with high frequency oscillations and a tank circuit for said oscillation generator in which said heating coil forms a part of said tank circuit, the combination of an inductance in said tank circuit comprising a winding formed of a helically wound conductor tube having suflicient inherent elasticity so that said winding springs back to its unrestrained axial length upon release of a force applied to reduce its axial length, spring means for maintaining the axial length of said winding at a minimum, and means responsive to the current flowing in the input circuit of said oscillation generator during a heating cycle to produce a variable force in opposition to said spring means so that the axial 13 length of said winding is varied to control the current flowing in said heating coil whereby said oscillation generator is utilized to its maximum efiiciency without exceeding a predetermined maximum load thereon.

13. In a high frequency heating device comprising an oscillation generator including a first electron discharge valve, a tank circuit connected to said first electron discharge valve including an inductor and a variable inductance, said variable inductance comprising a helical winding formed of an electrical conductor, means including an electric motor for varying the axial length of said winding, an energization circuit for said motor including a second electron discharge valve, and means responsive to a current characteristic of said first electron discharge valve for controlling the conductivity of said second electron discharge valve and consequently the output of said oscillation generator.

14. For use in a high frequency heating device of the type comprising an oscillation generator including a tank circuit comprising a heating winding, the combination of a variable inductance connected in said tank circuit comprising a helical coil formed of an electrical conductor, means including a direct current constant field electric motor having an armature winding for varying the axial length of said coil, an energization circuit for said motor including an electron discharge valve having its plate circuit connected in series with said armature winding, and means responsive to a current characteristic of said oscillation generator for controlling the conductivity of said electron discharge valve so as to tend to maintain constant the output of said oscillation generator.

15. For use in a high frequency heating device comprising a heating coil and an oscillation generator including a first electron discharge valve connected to supply said heating coil with high frequency oscillations in which a tank circuit is associated with said oscillation generatdr including said heating coil, the combination of a variable inductance connected in said tank circuit, said variable inductance comprising a helical winding formed of an electrical conductor, means including an electric motor for varying the axial length of said winding to control the output of said oscillation generator, an energization circuit for said motor including a second electron discharge valve having a plate, a cathode, and a control electrode, the plate circuit of said second electron discharge valve being connected in the energization circuit of said motor, and a control circuit for said control electrode including a portion of the grid circuit of said first electron discharge valve.

16. For use in a high frequency heating device comprising a heating coil and an oscillation generator including a first electron discharge valve connected to supply said heating coil with high frequency oscillations in which a tank circuit is associated with said oscillation generator including said heating coil, the combination of a variable inductance connected in said tank circuit, said variable inductance comprising a helical winding formed of an electrical conductor, means including an electric motor for varying the axial length of said winding to control the output of said oscillation generator, an energization circuit for said motor including a second electron discharge valve having a plate, a cathode, and a control electrode, the plate circuit of said second electron discharge valve being connected in the energization circuit of said motor, and a control circuit for said control electrode including a portion or the plate circuit of said first electron discharge valve.

17. For use in a high frequency heating device comprising a heating coil and an oscillation generator including an electron discharge valve connected to supply said heating coil in which a tank circuit connected to said electron discharge valve including said heating coil, the combination of a variable inductance connected in said tank circuit, said variable inductance comprising a helical winding formed of an electrical conductor, means including an electric motor for varying the axial length of said winding, spring means for opposing said electric motor, an energization circuit for said motor, and means for controlling the energization circuit of said motor in response to a current characteristic of said electron discharge valve to maintain substantially constant the output of said oscillation generator during a heating cycle.

HERBERT F. STORM.

REFERENCES CITED The following references are of record in the file of this patent:

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